This journal is c the Owner Societies 2011 Phys. Chem. Chem. Phys. Cite this: DOI: 10.1039/c0cp02798g VUV state-selected photoionization of thermally-desorbed biomolecules by coupling an aerosol source to an imaging photoelectron/photoion coincidence spectrometer: case of the amino acids tryptophan and phenylalanine Franc¸ois Gaie-Levrel,* a Gustavo A. Garcia, a Martin Schwell b and Laurent Nahon a Received 6th December 2010, Accepted 10th February 2011 DOI: 10.1039/c0cp02798g Gas phase studies of biological molecules provide structural and dynamical information on isolated systems. The lack of inter- or intra-molecular interactions facilitates the interpretation of the experimental results through theoretical calculations, and constitutes an informative complement to the condensed phase. However advances in the field are partially hindered by the difficulty of vaporising these systems, most of which are thermally unstable. In this work we present a newly developed aerosol mass thermodesorption setup, which has been coupled to a Velocity Map Imaging (VMI) analyzer operated in coincidence with a Wiley–McLaren Time of Flight spectrometer, using synchrotron radiation as a single photon ionization source. Although it has been previously demonstrated that thermolabile molecules such as amino acids can be produced intact by the aerosol vaporisation technique, we show how its non-trivial coupling to a VMI analyzer plus the use of electron/ion coincidences greatly improves the concept in terms of the amount of spectroscopic and dynamic information that can be extracted. In this manner, we report on the valence shell ionization of two amino acids, tryptophan and phenylalanine, for which threshold photoelectron spectra have been recorded within the first 3 eV above the first ionization energy using synchrotron radiation emitted from the DESIRS beamline located at SOLEIL in France. Their adiabatic ionization energies (IEs) have been measured at 7.40 Æ 0.05 and 8.65 Æ 0.02 eV, respectively, and their spectra analyzed using existing theoretical data from the literature. The IE values agree well with previously published ones, but are given here with a considerably reduced uncertainty by up to a factor of 5. The photostability of both amino acids is also described in detail, through the measurement of the state-selected fragmentation pathways via the use of threshold electron/ion coincidences (TPEPICO), with appearance energies for the different photofragments given for the vaporization temperatures studied, in correlation with the different molecular orbitals involved as identified from the Threshold Photoelectron Spectra (TPES). Introduction The study of fragile biomolecules in the gas phase is still a challenge in experimental physical chemistry. Not only is their volatility extremely low but also they can easily undergo chemical degradation when being heated. However, such studies are of utmost importance if one wants to measure directly the intrinsic physico-chemical properties of these species, which is of considerable interest in life sciences. Gas phase studies bring complementary information to those obtained by most condensed-phase techniques (X-ray crystallo- graphy or NMR) since they assess the gas phase chemical structure including conformers, electronic structure, and the photostability and dynamics of those systems. 1 Studying biomolecules in detail in the gas phase helps to mimic larger sub-structures of biopolymers. This so-called biomimetic concept is also important for quantum chemistry: the exact determination of intrinsic properties of small biological units isolated from their natural environment permits the modelling of larger biomolecular systems. 2 Furthermore, the gas phase can be directly relevant to in vivo conditions when one considers hydrophobic media. 1 a DESIRS beamline, Synchrotron SOLEIL, L’Orme des Merisiers, St Aubin 91192 Gif-sur-Yvette Cedex, France. E-mail: [email protected]b Laboratoire Interuniversitaire des Syste `me Atmosphe ´riques, UMR CNRS 7583, Universite´ Paris Est Cre ´teil et Universite ´ Paris Diderot, Institut Pierre Simon Laplace, 61 av. du Ge´ne ´ral de Gaulle, 94010 Cre ´teil Cedex, France PCCP Dynamic Article Links www.rsc.org/pccp PAPER Downloaded by Synchrotron Soleil on 14 March 2011 Published on 11 March 2011 on http://pubs.rsc.org | doi:10.1039/C0CP02798G View Online
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This journal is c the Owner Societies 2011 Phys. Chem. Chem. Phys.
Cite this: DOI: 10.1039/c0cp02798g
VUV state-selected photoionization of thermally-desorbed biomolecules
by coupling an aerosol source to an imaging photoelectron/photoion
coincidence spectrometer: case of the amino acids tryptophan
and phenylalanine
Francois Gaie-Levrel,*aGustavo A. Garcia,
aMartin Schwell
band Laurent Nahon
a
Received 6th December 2010, Accepted 10th February 2011
DOI: 10.1039/c0cp02798g
Gas phase studies of biological molecules provide structural and dynamical information on
isolated systems. The lack of inter- or intra-molecular interactions facilitates the interpretation of
the experimental results through theoretical calculations, and constitutes an informative
complement to the condensed phase. However advances in the field are partially hindered by the
difficulty of vaporising these systems, most of which are thermally unstable. In this work we
present a newly developed aerosol mass thermodesorption setup, which has been coupled to a
Velocity Map Imaging (VMI) analyzer operated in coincidence with a Wiley–McLaren Time of
Flight spectrometer, using synchrotron radiation as a single photon ionization source. Although it
has been previously demonstrated that thermolabile molecules such as amino acids can be
produced intact by the aerosol vaporisation technique, we show how its non-trivial coupling to a
VMI analyzer plus the use of electron/ion coincidences greatly improves the concept in terms of
the amount of spectroscopic and dynamic information that can be extracted. In this manner,
we report on the valence shell ionization of two amino acids, tryptophan and phenylalanine, for
which threshold photoelectron spectra have been recorded within the first 3 eV above the first
ionization energy using synchrotron radiation emitted from the DESIRS beamline located
at SOLEIL in France. Their adiabatic ionization energies (IEs) have been measured at
7.40 � 0.05 and 8.65 � 0.02 eV, respectively, and their spectra analyzed using existing theoretical
data from the literature. The IE values agree well with previously published ones, but are given
here with a considerably reduced uncertainty by up to a factor of 5. The photostability of
both amino acids is also described in detail, through the measurement of the state-selected
fragmentation pathways via the use of threshold electron/ion coincidences (TPEPICO), with
appearance energies for the different photofragments given for the vaporization temperatures
studied, in correlation with the different molecular orbitals involved as identified from the
Threshold Photoelectron Spectra (TPES).
Introduction
The study of fragile biomolecules in the gas phase is still a
challenge in experimental physical chemistry. Not only is their
volatility extremely low but also they can easily undergo
chemical degradation when being heated. However, such
studies are of utmost importance if one wants to measure
directly the intrinsic physico-chemical properties of these
species, which is of considerable interest in life sciences. Gas
phase studies bring complementary information to those
obtained by most condensed-phase techniques (X-ray crystallo-
graphy or NMR) since they assess the gas phase chemical
structure including conformers, electronic structure, and the
photostability and dynamics of those systems.1 Studying
biomolecules in detail in the gas phase helps to mimic larger
sub-structures of biopolymers. This so-called biomimetic
concept is also important for quantum chemistry: the exact
determination of intrinsic properties of small biological units
isolated from their natural environment permits the modelling
of larger biomolecular systems.2 Furthermore, the gas phase
can be directly relevant to in vivo conditions when one
b Laboratoire Interuniversitaire des Systeme Atmospheriques,UMR CNRS 7583, Universite Paris Est Creteil et Universite ParisDiderot, Institut Pierre Simon Laplace, 61 av. du General de Gaulle,94010 Creteil Cedex, France
Phys. Chem. Chem. Phys. This journal is c the Owner Societies 2011
Compared to other experiments using heated ovens to
vaporize biomolecules, the quantity of commercially available
or synthesized compounds used during an aerosol experiment
is considerably reduced. More precisely, only a few tens of
milligrams are used for a 24 hour experiment, a critical
advantage if one wants to study molecules difficult to synthesize
or commercially expensive.
In addition, we have demonstrated the ability to control the
thermal energy imparted to the neutral molecule during the
vaporization process by optimizing the TD temperature.
The present setup is currently being applied to the study of
photon-induced processes on natural and modified nucleo-
bases (NAB)47 showing the versatility of the method and its
relevance for future experiments on other biomolecules, such
as oligopeptides and for analytical chemistry using atmospheric-
pressure photoionization (APPI) for example.
With the use of the thermal desorption module, future
experiments on atmospheric aerosols chemical composition,
such as secondary organic aerosol (SOA) produced in a smog
chamber on the DESIRS beamline, are foreseen. The insertion
of a differential mobility analyzer (DMA) is also envisaged in
the short term, in order to size-select the SOAs prior to the
thermodesorption and obtain richer information on their
production and composition. The DMA will also be used for
the study of circular dichroism of nanometre-sized aerosol
particles and, more generally, for the study of the angular
distribution of photoelectrons emitted from nanoparticles,
such as those obtained for DOP nanoparticles.
Acknowledgements
We gratefully acknowledge financial support by the French
Groupe de Recherche ‘‘exobiologie’’ (which has become the
‘‘Societe Francaise d’Exobiologie’’ today) as well as the CNRS
Interdisciplinary programme ‘‘Evironnements planetaires et
Origine de la Vie’’. We thank Sydney Leach and Didier
Despois for helpful discussions. We are strongly indebted to
Jean-Francois Gil for his help in the design and the mounting
of the thermodesorber. We would also like to thank the
general technical staff of SOLEIL for running the facility.
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